Hi, I'm using Pixelmator 1.6.7 and have just got a new MacBook Pro (Retina, 13-inch, Early 2015) running MacOS High Sierra v 10.13.2. I know it's a bit old but since I don't do anything too complicated it's okay for me.

I transferred all my apps and files to the new machine and all is good but when using Pixelmator I am unable to paste images using Edit | Paste Cmd V.

I'm afraid that's a rather old version of Pixelmator (around 7 years old) and won't necessarily work well with the latest versions of macOS. Lots of things have changed since it was released, so issues like this are, unfortunately, unavoidable. In 2011, we released a paid upgrade to Pixelmator on the App Store in order to be able to continue developing it. The latest version of that app is available to try right here.